The present invention relates to a gas sensor control apparatus adapted for connecting to a gas sensor which includes at least one cell having a solid electrolyte layer and a pair of electrodes, and a heater, detecting a concentration of a specific gas component in a gas to be measured via the gas sensor, outputting the concentration detected to an outside thereof and controlling supply of an electric current to the heater, and relates to a method for controlling the gas sensor.
An oxygen sensor and an air-fuel ratio sensor are known as a gas sensor which is used for enhancing fuel economy and controlling combustion in an internal combustion engine for a vehicle such as an automobile. In addition, with enforcement of strict automobile emission control, there is a demand for reduction of nitrogen oxide (NOx) in exhaust gas. To meet the demand, a NOx sensor capable of directly detecting a NOx concentration has been proposed.
As such a gas sensor, there are known those sensors which include a plurality of cells each having an oxygen ion-conducting solid electrolyte (for instance, zirconia) body and a pair of electrodes formed thereon. Particularly, as a NOx sensor having the above structure, there is known a NOx sensor in which a first oxygen pumping cell and a second oxygen pumping cell are laminated. In this NOx sensor, a first measurement chamber into which a gas to be measured is introduced is defined and the first oxygen pumping cell exposed to the first measurement chamber controls an oxygen concentration in the gas at a constant value. The gas having the thus controlled oxygen concentration flows into a second measurement chamber communicated with the first measurement chamber. A constant electric voltage is applied to the second oxygen pumping cell exposed to the second measurement chamber to thereby decompose the NOx present in the gas in the second measurement chamber. As a result, an electric current corresponding to the NOx concentration flows through the second oxygen pumping cell, so that the NOx concentration is detected on the basis of the electric current.
For the purpose of performing accurate detection of the NOx concentration, it is necessary to activate each of cells by heating the NOx sensor to a predetermined activation temperature (for instance, 750° C. or higher). Therefore, the NOx sensor is provided with a unitary body constituted of a heater and each of cells and is connected to a controller which controls an electric current to be supplied to the heater. Such a NOx sensor control apparatus is disclosed in Japanese Patent Application Unexamined Publication No. 10-142194. The NOx sensor control apparatus of this conventional art measures an internal resistance (impedance) of one of cells constituting a NOx sensor, calculates a temperature of the NOx sensor (an element temperature) on the basis of the measured internal resistance and controls supply of an electric current to a heater such that the temperature calculated becomes equal to a target temperature. In addition to such a control function of supplying the current to the heater, the NOx sensor control apparatus ordinarily has functions of converting the current flowing in the second oxygen pumping cell to voltage, calculating a detection value of the NOx concentration (a NOx concentration conversion value) on the basis of the converted voltage, and outputting the detection value to an external engine control unit.
On the other hand, in such a gas sensor control apparatus, there will occur such a problem that the output detection value of the internal resistance of the cell is considerably different from an actual internal resistance value due to noise superimposed on a signal of the output detection value. In order to solve this problem, a technology of controlling a gas sensor is disclosed in Japanese Patent Application Unexamined Publication No. 2002-303601. In this conventional art, variation in amount of the internal resistance to be detected by the gas sensor control apparatus is limited to thereby reduce influence of noise and perform accurate control of the gas sensor.